1. Field of the Invention
The present invention relates to a rotary driving device used for a rotary actuator. The rotary driving device according to the present invention is used as an actuator for driving, e.g., a rotary valve.
2. Description of the Related Art
A conventional rotary driving device is, for example, constituted by stator magnetic poles, fixed in a housing, and a rotor of a permanent magnet rotatably supported by a shaft inside the magnetic poles. The polarities of the stator magnetic poles are reversed by an excitation coil, thereby rotating the rotor. The rotor has a cylindrical shape, and the respective stator magnetic poles are arranged on an identical circumference so that distances between inner end faces of the stator magnetic poles and a center of rotation of the rotor become the same. For this reason, lines of magnetic force from the rotor are distributed to be wider than an outer periphery thereof, and a magnetic attractive force between the stator magnetic poles and the rotor is weakened.
Therefore, when external rotation or vibration is applied to the excitation coil in a nonconductive state, the rotor is easily rotated and cannot maintain a stable rest position. In particular, when a rotary driving device of this type is compactly formed, i.e., into a shallow outer shape, the outer diameter of the rotor is decreased. Therefore, when the excitation coil is rendered nonconductive, the rest torque of the rotor becomes small and a stable rest position cannot be maintained. In addition, when the excitation coil is energized, only a small output torque can be obtained from the rotor, for the same reason as described above.